Anthony Joshua's scheduled fight against Dillian Whyte has been cancelled after the latter returned "adverse" findings from a random drugs test.
Promoter Matchroom added that a full investigation would be conducted after the announcement from the Voluntary Anti-Doping Association.
The bout was a rematch between the two due to take place on 12 August at the O2 Arena in London.
Whyte, 35, said he was "shocked and devastated" after the ruling.
"I can confirm without a shadow of doubt that I have not take the reported substance, in this camp or at any point in my life," he said.
"I am completely innocent and ask to be given the time to go through the process of proving this without anybody jumping to conclusions or a trial by media".
He added: "This is not the first time that I have been reported as having an adverse finding for a substance which I have not taken, and as I did last time I will again prove that I am completely innocent."
Former world champion Joshua has a long-running rivalry with Whyte, who defeated him in the amateur ranks, before Joshua went on to win an Olympic gold medal at London 2012.